As water goes through the freezing process, dissolved minerals in the “sump” or “make-up” water tend to increase in concentration. That is, in a given volume of water, the pure H2O will be the first to solidify; minerals that had been in the volume of the solidified water remain in solution and thereby cause the balance of liquid water to increase in mineral concentration. Dissolved minerals are usually referenced as “total dissolved solids” (TDS), and can be readily measured in units of parts per million (PPM).
In a commercial ice making machine (nugget or cube), relatively pure ice is formed and evacuated from the machine. As the ice leaves the machine, the concentration of the total dissolved solids (TDS) increases in the water within the evaporator or sump. (Note: the “sump” of a nugget ice machine is typically the evaporator barrel itself). The TDS level in the sump can increase greatly relative to the TDS level of the in-coming fresh water. This said, the sump TDS concentration is greatly dependent on the volume of available sump water; small water volumes can rapidly increase to maximum TDS levels in an ice machine. A short period of running (say, 10 minutes for example) could yield maximum sump TDS levels if the sump capacity is limited.
Conventional ice makers will periodically shut off the ice making process and drain the sump to evacuate the elevated TDS. Fresh water is then introduced to the system and the ice making process re-started. Cube-type ice makers will flush-out the system as often as every batch (perhaps every 20 minutes) while nugget ice machines may allow up to an hour of operation before dumping the high TDS water. Some commercial systems continuously measure the TDS and will interrupt operation for a flush cycle based on TDS limits.
Scale growth rates are somewhat related to TDS levels, although the prediction of scale formation typically combines additional factors including water pH, hardness, temperature, TDS composition, etc. TDS levels can be used to depict the magnitude of change of the water chemistry within the sump. Focus is placed on the differences between the fresh water and the sump water using TDS as an easy-to-measure metric. It is implied that if TDS increases as a multiple of the fresh water, some detrimental effects of the water chemistry could increase as well. Fresh water of a given chemistry is as “stable” as possible, and that any increase in TDS concentration could serve to “de-stabilize” or saturate the water toward detrimental effects, like scale growth.
To establish a perspective of common levels of TDS found in potable water supplies, some benchmarks are cited: fresh water of, say, 100 PPM (parts per million) can be referred as “good” water that does not need treatment, but water of, say, 600 ppm could form scale and often requires scale inhibiting treatments. TDS in commercial bottled water is found to be about 100 ppm. Some municipal water supplies attempt to limit fresh water TDS to 400 ppm, but will often treat water known to be scale prone. There are bands of geographic areas in the USA where water supplies can be measured to 600 ppm, with a few sporadic sites of very high TDS ranging between 800 to 1500 ppm, but these are rare findings that are usually related to seasonal or other extraneous circumstances. It is common to find water treatment systems in these areas of elevated TDS, because the added expense of treatment is offset by the extended operation of the ice makers.
In addition to TDS measurements, a common reference for predicting scale formation is the Langlier index. The derivation of the Langlier measurement is more complex than a simple TDS measurement because it combines additional factors that need to be measured (pH, hardness,
temperature, etc.). This said, water samples can be taken and a Langlier measurement can be derived. Langlier uses the mineral saturation level of the water as a predictor of scale growth. A level of “0” is considered neutral in scale formation; −1 has almost no chance of forming scale while +1 has a great propensity to form scale. The Langlier index of some water samples is cited below, again to show the magnitude of chemistry change of the water within an ice machine. The intent is to further highlight the magnitude of change of the sump water from fresh water.
However, within an ice maker, it is commonly observed that scale forms in just about all applications to lesser and greater extents based on water chemistry. Scale has been shown to aggressively form in an ice maker with water supplies of 200 ppm. The reason is as follows.
In an ice machine, as described above, water in the freezing chamber quickly reaches elevated mineral concentrations as a by-product of the freezing process. During operation, the concentration of TDS in the freezing chamber can easily be measured in the 1000's ppm range even though the fresh supply is so-called “good” water. Concentrations in the >6000 ppm range are measured in the freezing chamber of machines operating in high TDS fresh water supplies. Scale tends to rapidly form in the regions of the ice maker that tend to hold high concentrations of minerals, and conventional water treatments can tend to be only marginally effective at keeping scale from firming within the ice maker. Treatment systems that target the fresh water were not necessarily designed to manage the chemistry of the sump water.
An experiment shows the chemistry changes of the sump water in a scale prone application: Water samples were extracted and analyzed from an ice maker application site.
TDSLanglier IndexFresh Water469 ppm−0.2Evaporator Water drained out2226 ppm +1.3shortly after turning themachine offWater from Melted Ice716 ppm+0.2
It is clear that the water chemistry of the sump has completely changed into that which has greatly increased in TDS and has a strong potential to form scale; the Langlier index is very high.
A very critical aspect of TDS within a nugget ice machine is the fact that the concentration of TDS is not uniform in the freezing chamber. During operation of a nugget ice machine, high TDS water tends to be concentrated in the area of ice compression (at the discharge end of the auger) because of the confluence of several factors.
The actual squeezing of the ice occurs in this area; the purer ice leaves the machine from this area while the squeezed-out water remains behind.
Also, the local sump water in this area, with a given TDS level of “x” is forced to accept the squeezed-out water with the same TDS level, “x”. But because the volume of water in this area remains constant as ice is evacuated, the “local” sump TDS climbs to 2×, 3×, 4× etc. until a balance is struck between the fresh water TDS and the amount of TDS leaving the machine with the nugget.
Additionally, the available water volume is quite small in this area (this “local” sump area) owing to the fact that most of the auger open volume is filled with ice. The minute volume of water that is available to accept the squeezed-out high TDS water therefore tends to locally increase in TDS very rapidly to very high levels; levels that will readily form scale.
Furthermore, fresh water is conventionally fed in the end of the freezing chamber that is opposite the ice discharge end. Fresh water dilutes the TDS in the area in which it is fed but never can effectively reach the ice discharge end. Thus, experience has shown a high TDS concentration zone of a nugget ice maker to be in the region of the discharge end of the auger in prior art devices.
Fresh water in prior art applications enters from the rear of the machine at the reservoir and generally flows forward toward the ice discharge end. The high TDS water in the discharge end of the machine never has a chance to dilute. Only the conventional flushing process removes it from the machine. The flushing process unfortunately causes unwanted down-time, and water waste. Most often, a machine is caused to operate for a period with maximum TDS levels in the scale zone at the discharge end of the auger until flushing occurs.
Scale growth, particularly on the auger surface, in the discharge area of the machine, tends to “grab” the ice flakes. Rather than allowing the ice to freely move along the polished steel auger surface, the ice drags on the scaled surface. This leads to ice compaction on the auger and can soon render the ice maker inoperable because the ice can no longer reach the evacuation port of the machine. The only remedy is to clean the scale off of the auger with caustic cleaners or to disassemble and mechanically remove the scale from the auger. Often the owner is forced to install water treatment means to try to extend the intervals between needed cleaning, hut these can be somewhat futile since they are designed to manage the elements of the fresh water, not the excessive TDS levels found in the scale zone.
Because fresh water enters the rear area of the auger at the reservoir, TDS levels are much lower than the front (discharge end) area, and scale growth is minimal.
It is important to note that this elevated TDS phenomenon (the scale zone) occurs in all types of commercial nugget machines. The orientation of the freezing chamber is not the governing issue; the elevated TDS at the discharge end of the auger (horizontally or vertically disposed), coupled with the fresh water feed from the opposite end, renders the important auger working surface covered in scale.
Scale growth is related to the nugget hardness; harder nuggets tending to be more pure leave higher TDS behind in the scale zone. Harder nuggets tend to evacuate less water. The ice in the scale zone will be compacted more in order to drive the harder nuggets through the compression nozzles, and will be more susceptible to drag on the scale. Harder nuggets, however, are the preferred form because of the improved mechanical properties; they dispense better and they don't tend to clump up in the ice storage bin.
Nuggets that contain water with elevated TDS tend to degrade faster than those compacted within low TDS water. In a conventional machine, since ice is being compressed in the scale zone, the water carried out by the nugget is at the highest TDS level of all the water in the evaporator. TDS in the water can lower the freezing point, and therefore the nugget degrades. The best nugget characteristics are achieved when the “local” water (the water in the area of the ice during ice compression) is of the lowest possible TDS; i.e. fresh water.